To produce a ceramic electronic device, such as a CR built-in type substrate and a multilayer ceramic capacitor, normally, ceramic slurry composed of a ceramic powder and a binder (an acrylic resin, butyral resin, etc.), a plasticizer and an organic solvent (toluene, MEK) is prepared first. Next, the ceramic slurry is applied on a PET film by using the doctor blade method, etc., heated to be dried and, then, the PET film is peeled, so that a ceramic green sheet is obtained. Next, an internal electrode is printed on the ceramic green sheet and dried, the results are stacked and cut into a chip shape to obtain green chips, and the green chips are fired and, then, terminal electrodes are formed thereon, so that an electronic device, such as a multilayer ceramic capacitor, is produced.
To produce a multilayer ceramic capacitor, an interlayer thickness of sheets, on which internal electrodes are formed, is in a range of about 1 μm to 100 μm based on desired capacitance required as a capacitor. Also, in a multilayer ceramic capacitor, a part not formed with an internal electrode is formed on outer parts in the stacking direction of the capacitor chip.
A thickness of the dielectric layer corresponding to the part not formed with an internal electrode is several tens of μm to several hundreds of μm or so, and this part is formed by using a relatively thick ceramic green sheet without an internal electrode printed thereon. A thickness of a green sheet having an internal electrode printed thereon is relatively thin, so that when forming the outer parts by using the thin film green sheet, the number of stacking layers becomes large and the number of production steps increases, which results in an increase of the production costs.
The larger the number of the dielectric layers in one-chip capacitor, the higher the capacity, however, since a size of the chip is limited, the dielectric layer has to be thinner. The dielectric layer is obtained by forming a sheet shape by covering dielectric particles having a particle diameter in a sub-micron order with a resin (a binder), stacking the results and firing. Production of a thin green sheet leads to a thin dielectric layer.
Also, since a thin sheet is fragile and easily broken, the present inventors have proposed to use a polyvinyl butyral (PVB) resin as a highly strong binder resin for a resin for forming the sheet. As a result, a dielectric green sheet having a thickness of 2 μm or thinner is produced and handling without damaging has become possible.
A ceramic portion used in the multilayer chip capacitor has a cap part (outer layer) for forming outside of the chip in addition to the dielectric layers (internal layers) sandwiched by the internal electrode layers for obtaining a capacity. The inner layers are required to be thin as explained above, while the outer layers are required to have a certain degree of thickness to protect the internal structure. Also, a sheet property required to the inner layers is precision, smoothness and strength, while, the outer layers are required to have a property attaching importance to handleability, such as adhesiveness, air permeability and cutting property.
As explained above, required sheet properties are largely different and almost opposite between the outer layers and the inner layers. Accordingly, a sheet satisfying the property required to the outer layers cannot be generally formed by coating green sheet slurry specializing in the inner layers.
Therefore, methods of changing a composition of the resin and adding additives to compensate characteristics that the resin lacks, etc. have been known. For example, in the patent article 1, the sheet property is controlled by blending resins. In the patent article 2 below, a tackifier is added to improve adhesiveness of the sheet.
However, in the method in the patent article 1, binder resins have different compositions respectively in the inner layer green sheet and outer layer green sheet. As a result, binder removal reaction arises at different timing between the inner layers and outer layers in the binder removal step of heating green chips, so that it is liable that strength of the chips declines to result in cracks and other damages.
Also, in the method in the patent article 2, adhesiveness of the sheet is considered to be improved by adding a tackifier, however, since the tackifier is a kind of resin components, the same disadvantage as that in the patent article 1 may be concerned.
Patent Article 1: the Japanese Unexamined Patent Publication No. 2002-104878
Patent Article 2: the Japanese Unexamined Patent Publication No. 2000-133547